public
class
Hashtable
extends Dictionary<K, V>
implements
Map<K, V>,
Cloneable,
Serializable
| java.lang.Object | ||
| ↳ | java.util.Dictionary<K, V> | |
| ↳ | java.util.Hashtable<K, V> | |
 Known Direct Subclasses
|
|
Known Indirect Subclasses
|
This class implements a hash table, which maps keys to values. Any
non-null object can be used as a key or as a value.
To successfully store and retrieve objects from a hashtable, the
objects used as keys must implement the hashCode
method and the equals method.
An instance of Hashtable has two parameters that affect its
performance: initial capacity and load factor. The
capacity is the number of buckets in the hash table, and the
initial capacity is simply the capacity at the time the hash table
is created. Note that the hash table is open: in the case of a "hash
collision", a single bucket stores multiple entries, which must be searched
sequentially. The load factor is a measure of how full the hash
table is allowed to get before its capacity is automatically increased.
The initial capacity and load factor parameters are merely hints to
the implementation. The exact details as to when and whether the rehash
method is invoked are implementation-dependent.
Generally, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the time cost to look up an entry (which is reflected in most Hashtable operations, including get and put).
The initial capacity controls a tradeoff between wasted space and the
need for rehash operations, which are time-consuming.
No rehash operations will ever occur if the initial
capacity is greater than the maximum number of entries the
Hashtable will contain divided by its load factor. However,
setting the initial capacity too high can waste space.
If many entries are to be made into a Hashtable,
creating it with a sufficiently large capacity may allow the
entries to be inserted more efficiently than letting it perform
automatic rehashing as needed to grow the table.
This example creates a hashtable of numbers. It uses the names of the numbers as keys:
Developers
Hashtable<String, Integer> numbers
= new Hashtable<String, Integer>();
numbers.put("one", 1);
numbers.put("two", 2);
numbers.put("three", 3);
To retrieve a number, use the following code:
Integer n = numbers.get("two");
if (n != null) {
System.out.println("two = " + n);
}
The iterators returned by the iterator method of the collections
returned by all of this class's "collection view methods" are
fail-fast: if the Hashtable is structurally modified at any time
after the iterator is created, in any way except through the iterator's own
remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent
modification, the iterator fails quickly and cleanly, rather than risking
arbitrary, non-deterministic behavior at an undetermined time in the future.
The Enumerations returned by Hashtable's keys and elements methods are
not fail-fast.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
As of the Java 2 platform v1.2, this class was retrofitted to
implement the Map interface, making it a member of the
Java Collections Framework. Unlike the new collection
implementations, Hashtable is synchronized. If a
thread-safe implementation is not needed, it is recommended to use
HashMap in place of Hashtable. If a thread-safe
highly-concurrent implementation is desired, then it is recommended
to use ConcurrentHashMap in place of
Hashtable.
Public constructors | |
|---|---|
Hashtable(int initialCapacity, float loadFactor)
Constructs a new, empty hashtable with the specified initial capacity and the specified load factor. |
|
Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75). |
|
Hashtable()
Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75). |
|
Hashtable(Map<? extends K, ? extends V> t)
Constructs a new hashtable with the same mappings as the given Map. |
|
Public methods | |
|---|---|
void
|
clear()
Clears this hashtable so that it contains no keys. |
Object
|
clone()
Creates a shallow copy of this hashtable. |
V
|
compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)
Attempts to compute a mapping for the specified key and its current
mapped value (or |
V
|
computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)
If the specified key is not already associated with a value (or is mapped
to |
V
|
computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)
If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value. |
boolean
|
contains(Object value)
Tests if some key maps into the specified value in this hashtable. |
boolean
|
containsKey(Object key)
Tests if the specified object is a key in this hashtable. |
boolean
|
containsValue(Object value)
Returns true if this hashtable maps one or more keys to this value. |
Enumeration<V>
|
elements()
Returns an enumeration of the values in this hashtable. |
Set<Entry<K, V>>
|
entrySet()
Returns a |
boolean
|
equals(Object o)
Compares the specified Object with this Map for equality, as per the definition in the Map interface. |
void
|
forEach(BiConsumer<? super K, ? super V> action)
Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. |
V
|
get(Object key)
Returns the value to which the specified key is mapped,
or |
V
|
getOrDefault(Object key, V defaultValue)
Returns the value to which the specified key is mapped, or
|
int
|
hashCode()
Returns the hash code value for this Map as per the definition in the Map interface. |
boolean
|
isEmpty()
Tests if this hashtable maps no keys to values. |
Set<K>
|
keySet()
Returns a |
Enumeration<K>
|
keys()
Returns an enumeration of the keys in this hashtable. |
V
|
merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)
If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. |
V
|
put(K key, V value)
Maps the specified |
void
|
putAll(Map<? extends K, ? extends V> t)
Copies all of the mappings from the specified map to this hashtable. |
V
|
putIfAbsent(K key, V value)
If the specified key is not already associated with a value (or is mapped
to |
boolean
|
remove(Object key, Object value)
Removes the entry for the specified key only if it is currently mapped to the specified value. |
V
|
remove(Object key)
Removes the key (and its corresponding value) from this hashtable. |
boolean
|
replace(K key, V oldValue, V newValue)
Replaces the entry for the specified key only if currently mapped to the specified value. |
V
|
replace(K key, V value)
Replaces the entry for the specified key only if it is currently mapped to some value. |
void
|
replaceAll(BiFunction<? super K, ? super V, ? extends V> function)
Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. |
int
|
size()
Returns the number of keys in this hashtable. |
String
|
toString()
Returns a string representation of this Hashtable object in the form of a set of entries, enclosed in braces and separated by the ASCII characters ", " (comma and space). |
Collection<V>
|
values()
Returns a |
Protected methods | |
|---|---|
void
|
rehash()
Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. |
Inherited methods | |
|---|---|
From
class
java.util.Dictionary
| |
From
class
java.lang.Object
| |
From
interface
java.util.Map
| |
Hashtable (int initialCapacity,
float loadFactor)
Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
| Parameters | |
|---|---|
initialCapacity |
int:
the initial capacity of the hashtable. |
loadFactor |
float:
the load factor of the hashtable. |
| Throws | |
|---|---|
IllegalArgumentException |
if the initial capacity is less than zero, or if the load factor is nonpositive. |
Hashtable (int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).
| Parameters | |
|---|---|
initialCapacity |
int:
the initial capacity of the hashtable. |
| Throws | |
|---|---|
IllegalArgumentException |
if the initial capacity is less than zero. |
Hashtable ()
Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).
Hashtable (Map<? extends K, ? extends V> t)
Constructs a new hashtable with the same mappings as the given Map. The hashtable is created with an initial capacity sufficient to hold the mappings in the given Map and a default load factor (0.75).
| Parameters | |
|---|---|
t |
Map:
the map whose mappings are to be placed in this map. |
| Throws | |
|---|---|
NullPointerException |
if the specified map is null. |
Object clone ()
Creates a shallow copy of this hashtable. All the structure of the hashtable itself is copied, but the keys and values are not cloned. This is a relatively expensive operation.
| Returns | |
|---|---|
Object |
a clone of the hashtable |
V compute (K key,
BiFunction<? super K, ? super V, ? extends V> remappingFunction)
Attempts to compute a mapping for the specified key and its current
mapped value (or null if there is no current mapping). For
example, to either create or append a String msg to a value
mapping:
map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))
(Method merge() is often simpler to use for such purposes.)
If the remapping function returns null, the mapping is removed
(or remains absent if initially absent). If the remapping function
itself throws an (unchecked) exception, the exception is rethrown, and
the current mapping is left unchanged.
The remapping function should not modify this map during computation.
| Parameters | |
|---|---|
key |
K:
key with which the specified value is to be associated |
remappingFunction |
BiFunction:
the remapping function to compute a value |
| Returns | |
|---|---|
V |
the new value associated with the specified key, or null if none |
V computeIfAbsent (K key,
Function<? super K, ? extends V> mappingFunction)
If the specified key is not already associated with a value (or is mapped
to null), attempts to compute its value using the given mapping
function and enters it into this map unless null.
If the mapping function returns null, no mapping is recorded.
If the mapping function itself throws an (unchecked) exception, the
exception is rethrown, and no mapping is recorded. The most
common usage is to construct a new object serving as an initial
mapped value or memoized result, as in:
map.computeIfAbsent(key, k -> new Value(f(k)));
Or to implement a multi-value map, Map<K,Collection<V>>,
supporting multiple values per key:
map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
The mapping function should not modify this map during computation.
| Parameters | |
|---|---|
key |
K:
key with which the specified value is to be associated |
mappingFunction |
Function:
the mapping function to compute a value |
| Returns | |
|---|---|
V |
the current (existing or computed) value associated with the specified key, or null if the computed value is null |
V computeIfPresent (K key,
BiFunction<? super K, ? super V, ? extends V> remappingFunction)
If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
If the remapping function returns null, the mapping is removed.
If the remapping function itself throws an (unchecked) exception, the
exception is rethrown, and the current mapping is left unchanged.
The remapping function should not modify this map during computation.
| Parameters | |
|---|---|
key |
K:
key with which the specified value is to be associated |
remappingFunction |
BiFunction:
the remapping function to compute a value |
| Returns | |
|---|---|
V |
the new value associated with the specified key, or null if none |
boolean contains (Object value)
Tests if some key maps into the specified value in this hashtable.
This operation is more expensive than the containsKey method.
Note that this method is identical in functionality to
containsValue, (which is part of the
Map interface in the collections framework).
| Parameters | |
|---|---|
value |
Object:
a value to search for |
| Returns | |
|---|---|
boolean |
true if and only if some key maps to the
value argument in this hashtable as
determined by the equals method;
false otherwise. |
| Throws | |
|---|---|
NullPointerException |
if the value is null
|
boolean containsKey (Object key)
Tests if the specified object is a key in this hashtable.
| Parameters | |
|---|---|
key |
Object:
possible key |
| Returns | |
|---|---|
boolean |
true if and only if the specified object
is a key in this hashtable, as determined by the
equals method; false otherwise. |
| Throws | |
|---|---|
NullPointerException |
if the key is null |
See also:
boolean containsValue (Object value)
Returns true if this hashtable maps one or more keys to this value.
Note that this method is identical in functionality to contains (which predates the Map interface).
| Parameters | |
|---|---|
value |
Object:
value whose presence in this hashtable is to be tested |
| Returns | |
|---|---|
boolean |
true if this map maps one or more keys to the specified value |
| Throws | |
|---|---|
NullPointerException |
if the value is null |
Enumeration<V> elements ()
Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object to fetch the elements sequentially.
| Returns | |
|---|---|
Enumeration<V> |
an enumeration of the values in this hashtable. |
See also:
Set<Entry<K, V>> entrySet ()
Returns a Set view of the mappings contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation, or through the
setValue operation on a map entry returned by the
iterator) the results of the iteration are undefined. The set
supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Set.remove, removeAll, retainAll and
clear operations. It does not support the
add or addAll operations.
| Returns | |
|---|---|
Set<Entry<K, V>> |
a set view of the mappings contained in this map |
boolean equals (Object o)
Compares the specified Object with this Map for equality, as per the definition in the Map interface.
| Parameters | |
|---|---|
o |
Object:
object to be compared for equality with this hashtable |
| Returns | |
|---|---|
boolean |
true if the specified Object is equal to this Map |
See also:
void forEach (BiConsumer<? super K, ? super V> action)
Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.) Exceptions thrown by the action are relayed to the caller.
| Parameters | |
|---|---|
action |
BiConsumer:
The action to be performed for each entry |
V get (Object key)
Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the key.
More formally, if this map contains a mapping from a key
k to a value v such that (key.equals(k)),
then this method returns v; otherwise it returns
null. (There can be at most one such mapping.)
| Parameters | |
|---|---|
key |
Object:
the key whose associated value is to be returned |
| Returns | |
|---|---|
V |
the value to which the specified key is mapped, or
null if this map contains no mapping for the key |
| Throws | |
|---|---|
NullPointerException |
if the specified key is null |
See also:
V getOrDefault (Object key, V defaultValue)
Returns the value to which the specified key is mapped, or
defaultValue if this map contains no mapping for the key.
| Parameters | |
|---|---|
key |
Object:
the key whose associated value is to be returned |
defaultValue |
V:
the default mapping of the key |
| Returns | |
|---|---|
V |
the value to which the specified key is mapped, or
defaultValue if this map contains no mapping for the key |
int hashCode ()
Returns the hash code value for this Map as per the definition in the Map interface.
| Returns | |
|---|---|
int |
a hash code value for this object. |
See also:
boolean isEmpty ()
Tests if this hashtable maps no keys to values.
| Returns | |
|---|---|
boolean |
true if this hashtable maps no keys to values;
false otherwise.
|
Set<K> keySet ()
Returns a Set view of the keys contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation), the results of
the iteration are undefined. The set supports element removal,
which removes the corresponding mapping from the map, via the
Iterator.remove, Set.remove,
removeAll, retainAll, and clear
operations. It does not support the add or addAll
operations.
| Returns | |
|---|---|
Set<K> |
a set view of the keys contained in this map |
Enumeration<K> keys ()
Returns an enumeration of the keys in this hashtable.
| Returns | |
|---|---|
Enumeration<K> |
an enumeration of the keys in this hashtable. |
See also:
V merge (K key,
V value,
BiFunction<? super V, ? super V, ? extends V> remappingFunction)
If the specified key is not already associated with a value or is
associated with null, associates it with the given non-null value.
Otherwise, replaces the associated value with the results of the given
remapping function, or removes if the result is null. This
method may be of use when combining multiple mapped values for a key.
For example, to either create or append a String msg to a
value mapping:
map.merge(key, msg, String::concat)
If the remapping function returns null, the mapping is removed.
If the remapping function itself throws an (unchecked) exception, the
exception is rethrown, and the current mapping is left unchanged.
The remapping function should not modify this map during computation.
| Parameters | |
|---|---|
key |
K:
key with which the resulting value is to be associated |
value |
V:
the non-null value to be merged with the existing value
associated with the key or, if no existing value or a null value
is associated with the key, to be associated with the key |
remappingFunction |
BiFunction:
the remapping function to recompute a value if
present |
| Returns | |
|---|---|
V |
the new value associated with the specified key, or null if no value is associated with the key |
V put (K key,
V value)
Maps the specified key to the specified
value in this hashtable. Neither the key nor the
value can be null.
The value can be retrieved by calling the get method
with a key that is equal to the original key.
| Parameters | |
|---|---|
key |
K:
the hashtable key |
value |
V:
the value |
| Returns | |
|---|---|
V |
the previous value of the specified key in this hashtable,
or null if it did not have one |
| Throws | |
|---|---|
NullPointerException |
if the key or value is
null |
See also:
void putAll (Map<? extends K, ? extends V> t)
Copies all of the mappings from the specified map to this hashtable. These mappings will replace any mappings that this hashtable had for any of the keys currently in the specified map.
| Parameters | |
|---|---|
t |
Map:
mappings to be stored in this map |
| Throws | |
|---|---|
NullPointerException |
if the specified map is null |
V putIfAbsent (K key,
V value)
If the specified key is not already associated with a value (or is mapped
to null) associates it with the given value and returns
null, else returns the current value.
| Parameters | |
|---|---|
key |
K:
key with which the specified value is to be associated |
value |
V:
value to be associated with the specified key |
| Returns | |
|---|---|
V |
the previous value associated with the specified key, or
null if there was no mapping for the key.
(A null return can also indicate that the map
previously associated null with the key,
if the implementation supports null values.) |
boolean remove (Object key, Object value)
Removes the entry for the specified key only if it is currently mapped to the specified value.
| Parameters | |
|---|---|
key |
Object:
key with which the specified value is associated |
value |
Object:
value expected to be associated with the specified key |
| Returns | |
|---|---|
boolean |
true if the value was removed |
V remove (Object key)
Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.
| Parameters | |
|---|---|
key |
Object:
the key that needs to be removed |
| Returns | |
|---|---|
V |
the value to which the key had been mapped in this hashtable,
or null if the key did not have a mapping |
| Throws | |
|---|---|
NullPointerException |
if the key is null
|
boolean replace (K key,
V oldValue,
V newValue)
Replaces the entry for the specified key only if currently mapped to the specified value.
| Parameters | |
|---|---|
key |
K:
key with which the specified value is associated |
oldValue |
V:
value expected to be associated with the specified key |
newValue |
V:
value to be associated with the specified key |
| Returns | |
|---|---|
boolean |
true if the value was replaced |
V replace (K key,
V value)
Replaces the entry for the specified key only if it is currently mapped to some value.
| Parameters | |
|---|---|
key |
K:
key with which the specified value is associated |
value |
V:
value to be associated with the specified key |
| Returns | |
|---|---|
V |
the previous value associated with the specified key, or
null if there was no mapping for the key.
(A null return can also indicate that the map
previously associated null with the key,
if the implementation supports null values.) |
void replaceAll (BiFunction<? super K, ? super V, ? extends V> function)
Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. Exceptions thrown by the function are relayed to the caller.
| Parameters | |
|---|---|
function |
BiFunction:
the function to apply to each entry |
int size ()
Returns the number of keys in this hashtable.
| Returns | |
|---|---|
int |
the number of keys in this hashtable. |
String toString ()
Returns a string representation of this Hashtable object in the form of a set of entries, enclosed in braces and separated by the ASCII characters ", " (comma and space). Each entry is rendered as the key, an equals sign =, and the associated element, where the toString method is used to convert the key and element to strings.
| Returns | |
|---|---|
String |
a string representation of this hashtable |
Collection<V> values ()
Returns a Collection view of the values contained in this map.
The collection is backed by the map, so changes to the map are
reflected in the collection, and vice-versa. If the map is
modified while an iteration over the collection is in progress
(except through the iterator's own remove operation),
the results of the iteration are undefined. The collection
supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Collection.remove, removeAll,
retainAll and clear operations. It does not
support the add or addAll operations.
| Returns | |
|---|---|
Collection<V> |
a collection view of the values contained in this map |
void rehash ()
Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. This method is called automatically when the number of keys in the hashtable exceeds this hashtable's capacity and load factor.